In the hair sector, two types of dyeing may be distinguished.
The first is semi-permanent or temporary dyeing, or direct dyeing, which involves dyes that are capable of giving the natural coloration of the hair a more or less pronounced color change that may be resistant to shampoo-washing several times. These dyes are known as direct dyes; they may be used with or without an oxidizing agent. In the presence of an oxidizing agent, the aim is to obtain lightening direct dyeing. Lightening dyeing is carried out by applying to the hair an extemporaneous mixture of a direct dye and an oxidizing agent, which makes it possible especially to obtain, by lightening the melanin of the hair, an advantageous effect such as a uniform color in the case of gray hair, or to bring out the color in the case of naturally pigmented hair.
The second is permanent dyeing or oxidation dyeing. This is performed with “oxidation” dyes comprising oxidation dye precursors and couplers. Oxidation dye precursors, commonly called “oxidation bases”, are compounds that are initially uncolored or only weakly colored, which develop their dyeing power on the hair in the presence of oxidizing agents that are added at the time of use, leading to the formation of colored and dyeing compounds. The formation of these colored and dyeing compounds results either from an oxidative condensation of the “oxidation bases” with themselves or from an oxidative condensation of the “oxidation bases” with coloration modifiers commonly called “couplers” and which are generally present in the dye compositions used in oxidation dyeing.
The variety of molecules used, which consist on the one hand of the “oxidation bases” and on the other hand of the “couplers”, allows a very wide range of colors to be obtained.
To further vary the shades obtained with said oxidation dyes, or to enrich them with glints, direct dyes may be added thereto.
The “permanent” coloration obtained with these oxidation dyes must moreover satisfy a certain number of requirements. Thus, it must be able to produce shades in the desired intensity and must show good resistance to external agents (light, bad weather, washing, permanent-waving, perspiration and rubbing).
The dyes must also be able to cover gray hair, and, finally, they must be as unselective as possible, i.e. they must produce the smallest possible color differences along the same keratin fiber, which may in fact be differently sensitized (i.e. damaged) between its end and its root.
To localize the oxidation dye product to application to the hair, so that it does not run onto the face or beyond the areas that it is intended to dye, in addition to the use of conventional thickeners, such as crosslinked polyacrylic acid, hydroxyethylcelluloses, waxes, and mixtures of appropriately selected nonionic surfactants with an HLB (Hydrophilic-Lipophilic Balance) value, associative polymers of anionic, nonionic or cationic type have also been used.
Associative polymers of anionic, nonionic or cationic type represent a major advance in the search for a solution to this problem.